1. Field
Disclosed herein is a method for the leaching of oxidized ores and in particular zinc ores. The ore, wherein the valuable metals are at least partially in silicate form, are routed to an acidic leaching stage in conditions where the silicate decomposes and the valuable metal ion comes into the solution. During leaching the silicate ion first dissolves, but simultaneously decomposes and is precipitated as silica.
2. Description of Related Art
The majority of the world's reserves of zinc occur in so called oxidized ore, where the zinc is bound to silicates and carbonates. Such are for instance smithsonite (ZnCO3) and willemite (2ZnO.SiO2) and mixtures of these minerals. Zinc-containing siliceous slags also exist, which originate mainly from the production of lead. Thus in the text the terms ore or raw material are also used to mean other zinc-containing siliceous raw materials than actual ores. Industrially these ores are utilized both pyrometallurgically and hydrometallurgically. Hydrometallurgical utilization occurs by leaching. The carbonate portion does not cause any major difficulties in leaching, but on the other hand the siliceous fraction is hard to control. Various processes have been proposed to overcome these difficulties.
U.S. Pat. No. 4,148,862 describes how the problem of siliceous zinc ores relates to the precipitation of silica, SiO2, and especially the morphology of silica. In industrial operation, the precipitation rate in particular is problematic. In the US publication in question, there is a diagram that illustrates the dependence of the stability of silica gel in an aqueous solution on the pH value. This shows that the precipitation rate of silica is at its best in a very acidic solution and again in a pH range of 3-5, but that a pH of 2 is very unfavorable for precipitation. It is clear from the patent text that the problems arising in connection with a siliceous zinc ore are quite different from those in normal zinc processes, in which the raw material is a sulphidic concentrate.
At least three ways have been proposed to combat the problem of a siliceous zinc concentrate. The first of these is described in for instance U.S. Pat. No. 3,656,941, where the leaching of a siliceous material performed in acidic conditions is rapid, so that the decomposed silica remains in solution. The solution is transferred to a second stage, where the SiO2 is precipitated. Precipitation occurs by neutralizing the solution with a calcine, limestone, lime or other suitable neutralizing agent.
When neutralization takes place as a continuous process, as described in U.S. Pat. No. 3,656,941, the solution can be filtered well even though the dissolved silica is up to 50 g/l. On the other hand however, it is known that the solution is unstable with regard to silica and sooner or later the silica will start to precipitate from the solution as an unfilterable gel. Therefore such a process is risky on industrial scale since one must always be prepared for stoppages that over time lead to the uncontrollable precipitation of silica.
Another treatment method of siliceous zinc ore is described in U.S. Pat. No. 3,954,937, where in the siliceous material leaching the pH value of the solution is lowered gradually so that at the end of the leach it is about 1.5 i.e. the H2SO4 content is about 1.5-15 g/l. The reduction of the pH value is carried out so slowly that the silica is able to precipitate. In a continuous process the siliceous material is brought to the first reactor and the acid is added to each reactor in the direction of the slurry flow. The drawback of this method is that the process requires the extremely careful regulation of each reactor. If too much acid goes into one reactor, silica is precipitated uncontrollably and a precipitate difficult to filter is obtained.
The above-mentioned U.S. Pat. No. 4,148,862 describes a third method, where the leaching of a siliceous material is performed in a single reactor and the pH is held constant (maximum 2.5) throughout the duration of the leach. Residence time is prolonged until the silica is able to precipitate. Although it is not stated in detail in the patent, it is evident that the solution coming from this acidic leach must be neutralized after filtration before it can be transferred forward. In order for the amount of neutralizing agent to be kept to the minimum, it is of course worth keeping the pH in the leaching and precipitation stage of the siliceous material as high as possible.